There are a number of applications in which metal or metal alloy paritcles must be bonded together and to a metal substrate. An effective method for joining the particles involves sintering, that is, heating without melting the particles, to bond the particles by diffusion of their constituent material. The diffusion bonding forms "necks" which join the particles to themselves and to a metal substrate. Sintering presently requires moderate to high heating of the particles in a vacuum or inert atmosphere for a long period of time. However, the lengthy treatment at a sufficiently high temperature begins to degrade the metal substrate through transformation of its grain microstructure. The substrate begins to exhibit decreased fatigue crack initiation resistance and other weaknesses.
One application of sintered particles involves formation of porous coating for prosthetic devices. There is a large and increasing demand for prosthetic devices such as replacement joints. Serious arthritic problems, for example, are often alleviated through surgery involving implantation of a metal joint into the bone of the patient to replace the natural joint. The metal joint is held by a shaft which was often anchored into the bone using an acrylic bone cement such as a grout. However, patients receiving such surgery are becoming younder in age and are living longer and more active lives. The bone cement used to anchor the shaft may not be sufficiently durable.
To improve adhesive performance and in certain instances to avoid the use of bone cement, one construction of prosthetic devices utilizes a coarse powder sintered onto a shaft made from a strong alloy such as Ti-6Al-4V alloy or cobalt alloy. In the latter case the resultant porous coating enables the living bone surrounding the shaft to grow into the pores and firmly lock the prosthesis in place. The powder may be of the same composition as the metal substrate, e.g. Ti6Al4V alloy or cobalt alloy having a composition as defined by the specification ASTM F-75; pure titanium powder (CpTi) is also used to coat titanium alloy prosthesis stems. The particles are initially secured to the substrate using an organic binder, which volatilizes below the sintering temperature. For sintering to Ti6Al4V, particles and binder are placed in a furnace at 1250.degree. C. for three hours in a high vacuum , e.g., 10.sup.-6 mm Hg. However, the Ti6Al4V alloy undergoes a solid state phase transformation at temperatures above 1000.degree. C. The microstructure is transformed to a form which lowers fatigue resistance. In addition, the diffusion bonds developed in the standard sintering process tend to have small areas of contact, that is, there are small radii of curvature of the fillet formed in the "notch" or neck region between particle and substrate. These bonds thus may be inherently weak; if the hard vacuum is not maintained the bond may be weakened further by embrittlement due to contamination by oxygen and nitrogen (air), and by carbon from back-streamed oil vapor from the pumping system. These weaknesses may cause the prosthesis to fail after implantation in a patient.